We have a 5-month-old female lynx cat that seems to be very healthy, but we can feel her ribs and backbone. Is this normal?

You mention "lynx" cat, but this word refers to a few different breeds. There are the desert lynx and the highland lynx, for example. A "domestic lynx" refers to a cross between a small bobcat and Canadian lynx with domestic cats. They are gorgeous, fascinating felines, so congratulations on bringing such a beauty into your home.
Ideally, you should be able to feel your cat's ribs, but there should be a slight covering of fat over them. In other words, you shouldn't just feel skin and bones. You should be able to observe a waist behind its ribs too. As kittens grow, they generally follow certain weight patterns. At eleven weeks, a normal kitten ideally weighs about 2.5 pounds. This jumps to about 4.5 pounds at 20 weeks, given the body changes that occur. By 50 weeks, your cat should weigh around 7 pounds.
It's normal to feel a cat's ribs and backbone, but something else about your cat's appearance might be triggering your concern. We often hear about problems associated with overweight cats, but being underweight can be a symptom of underlying health issues, too. Get your cat checked out by a veterinarian to rule out such problems.